Telecommunication satellites are generally provided with antennae which permit the generation of one or more beams having an optimized directivity, in order to deliver a footprint which ensures the coverage of specified service zones. These service zones are generally defined and fixed at the satellite design stage, and cannot be adjusted in service during the lifetime of the satellite.
Flexible antennae are antennae which are capable of synthesizing one or more beams, whereby each beam is defined by a law of illumination which is adjustable in accordance with requirements during the service life of the satellite. The synthesis of one or more beams is achieved by the amplitude control and/or phase control of each elementary radiating feed of the antenna. The capacity for the modification of the position and the shape of the beam in orbit is particularly useful for the adjustment of the footprint in response to a change of requirements, or in order to generate a directional beam, or to ensure anti-jamming capabilities.
A first solution for the achievement of a flexible footprint could be the use of an active DRA (Direct Radiating Array) antenna. This type of antenna is not fitted with a reflector, and comprises an array of radiating feeds associated with a BFN (Beam-Forming Network), attenuators, phase shifters and amplifiers. The DRA permits the synthesis of reconfigurable beams, but requires a large number of radiating feeds and, in consequence a large number of amplitude and phase controls, thereby necessitating a substantial quantity of on-board hardware, both for the BFN and for its electronic control and regulation device. This type of antenna is therefore particularly complex and voluminous, expensive and heavy, such that its use is frequently limited to military applications.
A second solution could be the use of a FAFR (Focal Array Fed Reflector) antenna, comprised of an array of radiating feeds accommodated at the focal point of an unshaped parabolic reflector. The coverage achieved by this antenna is a direct image of the focal spot of the array shape, whereby the position of the feeds is directly linked to the shape of the area to be covered. Each feed contributes to an element of the footprint, described as a cell. Consequently, there is a direct relationship between the size in area of the footprint, also described as coverage, and the number of radiating feeds, which may become very large when very extensive coverage is required, thereby resulting in problems in the fitting-out of a satellite. This type of antenna may be suitable for applications which are restricted to certain frequency bands and ground coverage of limited size. Moreover, if these areas of ground coverage are to be adjusted, this antenna requires the extinction or activation of a certain number of feeds and thereafter, generally, a re-optimization of the laws of amplitude and/or of phase, which necessitates the use of a matrix of switches and a large number of controls. The BFN architecture is therefore particularly complex, with a consequent mass, volume and cost. Moreover, the resolution of the antenna, which is directly linked to the size of the feeds, and the reconfiguration capacity, which is directly linked to the capacity of the BFN, are limited. It is possible to reduce the complexity of this type of antenna by using a reflector with a shaped surface, which permits the expansion of the size of the elementary beams generated by each feed, and a reduction in the number of feeds required for generating a beam which ensure the ground coverage, and in the number of corresponding controls. However, the resulting antenna is still highly complex, expensive and voluminous.
Although these various known antennae feature capacities for flexibility, they all have a major disadvantage, in that they are not ideal for conventional telecommunication functions.